1. Field of the Invention
The invention relates to an optical film.
2. Description of Related Art
With advancement in optoelectronic industries in the recent years, requirements for shape of an exit beam from a light source module have gradually increased. Accordingly, various types of optical films have been developed to comply with the requirements for the beam shape. These optical films include diffusion sheets and brightness enhancement films (BEFs) such as prism sheets, lens arrays, and so on. More specifically, a prism sheet acting as a BEF employs light emitting prism surfaces of a plurality of rod-shaped prisms to select among incident light rays in different incident directions. Typically speaking, a light beam entering a bottom portion of the rod-shaped prism at an angle of approximately 60° passes through the prism due to refraction effects of the rod-shaped prism, and this beam exits out of the prism in a direction approximately perpendicular to the bottom portion of the rod-shaped prism. Moreover, the light beam having an incident angle significantly greater than or smaller than 60° may not pass through the prism due to the total reflection effect of the light emitting prism surface. The light beam is reflected by the prism back to an optical device located below the prism sheet and reused.
Accordingly, the prism sheet achieves a light condensing effect for configuring the exit light beam to be substantially forward-emitted.
However, a crest line at the top portion of the rod-shaped prism and a valley line formed between two adjacent rod-shaped prisms are clearly observable and disposed at a regular interval. Therefore, a moiré phenomenon easily occurs in a display panel (e.g. a liquid crystal display panel) disposed above the prism sheet or the BEF, thereby affecting the display quality of the display panel. Besides, the rod-shaped prism may form a sharp corner at the crest line, and the sharp corner is prone to be worn by or to scratch the adjacent optical devices. Additionally, since each of the rod-shaped prisms has the same height, the prism sheet may locally adhere to an adjacent optical film rather easily, thereby giving rise to local abnormalities in the displayed image.
Taiwan Patent No. 1302989 discloses an optical film having a transparent substrate, a structural layer, and a plurality of light guide units. The structural layer may be a light condensing unit (e.g. a prism structure) disposed on the transparent substrate. The light condensing unit has a peak line and two surfaces. The light guide units are disposed on the peak line or the two surfaces. The light guide units may have a tapered shape, a circular arch shape, a circular arc shape, or a prism shape.
Furthermore, Taiwan Patent No. M352033 discloses an optical film having a microstructure layer, and the microstructure layer has a columnar structure. Taiwan Patent No. I274896 discloses a light condensing sheet including a substrate and a plurality of curved, rod-shaped prism units. Taiwan Patent Publication No. 200712655 discloses an optical film having a surface with a plurality of irregular prism blocks. Taiwan Patent Publication No. 200819855 discloses an optical film, a surface of the optical film has a plurality of light guide microstructures, and each of the light guide microstructures has two peaks or more. U.S. Pat. No. 7,278,771 discloses an optical film, and a surface of the optical film has curved rod-shaped prism structures. U.S. Pat. No. 6,845,212 discloses an optical film, and a surface of the optical film has wave-shaped, rod-shaped prism structures.
Taiwan Patent Publication No. 200912390 discloses an optical film including a substrate and a plurality of rod-shaped prisms disposed on the substrate. Several sphere-like lenses are disposed on crest lines of the rod-shaped prisms, and the short diameter (the short axis) of each of the sphere-like lenses is greater than 1 μm and less than 60 μm. Taiwan Patent Publication No. 200707397 discloses an optical film, and the sides of a rod-shaped prism of the optical film have protruding particles. Taiwan Patent Publication No. 200811479 discloses an optical film, and a substrate of the optical film has a prism layer. The prism layer includes a plurality of rod-shaped prisms, and sides of each of the rod-shaped prisms have a plurality of micro-lenses. U.S. Patent No. 20090311486 discloses a prism sheet having a rod-shaped prism, and surfaces of the rod-shaped prism have diffusion particles.